Winches, for example car or sailboat winches, are known in which a length of cable is attached at one end to a drum and an end of the cable can be unwound from and rewound onto the drum. Manual effort, or an automated cable feeding device, is typically required to guide the cable onto the drum. The cable is moved along the surface of the drum as it is wound in order to form adjacent coils on the surface of the drum and the cable is typically laid over itself in layers.
As the drum rotates, the coils move in space as if they were moving along a spiral. The drum is therefore required to have sufficient axial length to accommodate this movement of the cable. As an alternative to, or in conjunction with, increasing the axial dimension of the drum, it is known to allow the cable to be laid over itself, in effect increasing the axial length of the drum available to receive the cable. However, winches of this known type cannot be used in applications in which the total length of the line is greater than that which can be accommodated by the drum.
A further problem of such known winches is that the coils in an upper layer of cable can become embedded into the gaps between the coils of a lower layer of cable. This causes friction between the layers resulting in wear of the cable. It also impedes smooth operation and limits the load rating of the winch. Coiling the cable onto the drum also typically involves slippage (in some cases deliberate) of the cable across the drum surface or an underlying layer of cable. This results in frictional wear of the cable, mechanical shock to the system and noise. In deepwater winching applications (e.g. ocean bottom perforation or pipe laying applications) where the system is partially dry and partially wet due to components of the system being above and below the water line, variations in the co-efficient of friction of the cable relative to the drum make slippage even more undesirable.
In use of known winches in which the coils of cable are laid adjacent one another on the drum and laid over each other in layers, the required length of the drum is dictated in part by the length of cable to be unwound from and wound onto the drum. There is often a need to find a balance between the length of cable required for a particular application and the associated physical size of the winch. Known winches are also not optimally designed for heavy load, deepwater applications such as marine deep water operations which can require very long runs of cable (e.g. 4000 metres) for which a drum hundreds of metres long would be required to receive the cable. A known way to improve marine systems is to segment the cable and raise and lower it in sections but this method is complicated and unsuitable for high load applications.
US patent publication no. US2010/0059620 to Crawford provides an improved winch in which the drum has a complex shaped surface and the cable is wound onto the drum by a spooling head so that radially adjacent layers of flexible member are laid such that they are non-parallel to one another. This is intended to reduce interference between layers of spooled cable and lead to smoother operation. However, this is a complicated device having a limited load rating and requiring a slidable spooling head to change the position of the flexible member on the drum. It also requires superposition of coils of cable which are likely to interfere with one another and cause unwanted cable wear.
U.S. Pat. No. 4,351,197 to Carson provides a precision positioning apparatus having a driving element in the form of a rotary lead screw and a driven element in the form of a rotary drum. The lead screw is driven by a motor and moves axially relative to the drum in order that the portion of wraps of the cable on the drum remain in axially fixed positions as the lead screw rotates the drum. This is achieved by a complicated, delicate and inefficient mechanism that permits only limited rotation of the drum in either direction, making it unsuitable for applications involving a high load or long runs of cable.
U.S. Pat. No. 5,105,672 to Carson et al. also provides a rotary drive apparatus comprising a screw element driving member and driven drum member coupled by a cable. The number of turns of the drum is determined by the length of the screw and is therefore very limited. There are also significant stresses on the cable as it moves between threads on the screw.
U.S. Pat. No. 5,562,556 to Carson provides a continuous cable rotary drive apparatus having a drum and a capstan with parallel axes of rotation and coupled by an endless or continuous cable that is tensioned by a tensioning device. The device is intended to enable continuous rotation of the capstan and drum in either direction. However, because the cable grooves on the drum and capstan are offset by half the separation of the grooves, the cable is subject to unwanted stresses and friction of the cable as the drum and capstan rotate and the cable slips and jumps out of the grooves on the drum and into the grooves on the capstan and vice versa.
German patent number DE10117077C1 discloses an assembly for clearing dung from an animal stall having a manure scraper which is moved to and fro by a continuous cable around deflection rollers, through a cable drive. In a first embodiment the cable is allowed to slip on the surface of a deflecting pulley, resulting in significant wear of it. In a second embodiment with multiple turns of the cable on the traction drum, the assembly includes grooved drive wheels angled to one another around which the cable is looped, reproducing in part a standard traction winch arrangement. The grooves guide the cable from one drive wheel to the other and ensure that the cable cannot escape from the drive wheels as it passes from one wheel onto the other. The presence of the grooves, which change the angle of the cable as it passes from one wheel onto the other, inevitably leads to rubbing of the cable and associated wear. Also in this second embodiment the assembly also might permit some movement of the cable across the surface of the drive wheels as they rotate resulting in cable wear.